Speaker devices have become prevalent in home audio equipment and in-vehicle audio equipment, etc., and are widely adopted in personal computers and mobile terminals such as cell phones. In recent years, high-resolution sound sources in which sound information of a frequency band other than a zone of audibility, inaudible to the human ear,—are stored have attracted attention, and speaker devices adaptable to these high-resolution sound sources have also been actively developed.
Such a speaker device generally includes a magnetic circuit including a yoke, a magnet, a plate, and a center pole, etc., and a vibrating body including a voice coil, a diaphragm, and a frame, etc. When the speaker device operates, the voice coil vibrates according to a change in current flowing in the voice coil in a magnetic field made by the magnet, and further, the diaphragm connected to the voice coil vibrates to radiate sound waves to the outside.
For the magnetic circuit of the speaker device, a conductive material such as iron with high permeability is mainly used. Therefore, it is known that when a current is flowed in the voice coil, an AC magnetic field crosses the magnetic circuit by a magnetic field generated from the voice coil and generates an eddy current in a direction to obstruct a change in the magnetic circuit.
This eddy current causes a distortion of the current flowing in the voice coil, so that there is a risk that the eddy current blocks responsiveness of the voice coil and causes deterioration sound quality.
In order to reduce such an eddy current that causes deterioration in sound quality of a speaker device, for example, in Patent Literature 1, technology to reduce generation of an eddy current by making at least a portion of a yoke constituting a magnetic circuit, proximal to a magnet, of an iron powder bond has been proposed.
In detail, as shown in FIG. 7, a magnetic gap 313 is defined by an inner circumferential surface of a magnet 305 and an outer circumferential surface of a yoke 304, a voice coil 319 is inserted in this magnetic gap 313, and a portion of the yoke 304 facing the magnetic gap 313 is made of an iron Powder bond 321.
The iron powder bond 321 has higher volume resistivity and generates a higher electric resistance as compared with normal iron, so that an electric resistance at a peripheral portion of the voice coil 319 can be made larger relative to other portions. Therefore, an eddy current that is generated in the peripheral portion of the voice coil 319 can be minimized, responsiveness of the voice coil 319 to an electric signal is improved, and sound quality of the speaker device is improved.
Patent Literature 2 discloses a technology to suppress generation of an eddy current by not disposing a center pole that is considered to be a cause of generation of an eddy current, and is disposed on an inner circumferential side of a bobbin around which the voice coil is wound.
In detail, as shown in FIG. 8, ring-shaped plates 403a and 403b are disposed via a small gap on the outer circumferential side of the voice coil 419 wound around the bobbin 418 made of a non-magnetic material, and a magnet 405 that also has the same ring shape is disposed between these plates 403a and 403b. While an inner diameter of the magnet 405 is equal to inner diameters of the plates 403a and 403b, the outer diameter of the magnet 405 is larger than outer diameters of the plates 403a and 403b. Accordingly, it becomes easy for a magnetic flux that passes through both end faces in the axial direction of the magnet 405 to pass through the inner circumferential surfaces of the plates 403a and 403b. Therefore, a flux content that crosses the gap can be increased, so that even if a center pole is absent, a sufficient magnetic flux can be made to pass through the voice coil 419, and an eddy current that is generated by the presence of a center pole can be suppressed.